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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 14th Sep 2020, 12:17 pm   #1
retailer
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Default Rewinding a variac

I know this subject has been discussed at times in this and other forums - I damaged this variac many years ago while working in the physics lab at a senior secondary college. I should have realised the Tesla coil build (powered by a 15KV smoke precipitator transformer) that I was working on was too much for my weakling 260 watt variac and I should have left it at home but I plowed ahead anyway with obvious results.

Now many years down the track I'm looking for a variac to build into a test bench power supply - essentially a power point where a variac and dim bulb current limit can be switched in or out of the circuit and to top it off a double pole isolating switch, for those times when you need to make changes to a circuit and you get sick of unplugging each time before reaching for the soldering iron.

I did try in the past to do some repairs to the variac but could not trace the wires with sufficient accuracy to make it work so it has sat in a box waiting for today - I have nothing to lose except some wire if it is not a success. I measured the circumfrence and working on the wire diam. I calc the turns to be 1060, however I'm not going to count the turns but the plan is to unwind the wire onto my timber 'shuttle', measure the total length, fill the shuttle with new wire of same length (+ an extra meter or two), rewind, anchor the turns with varnish or epoxy.

Once the varnish is dry I plan to lay a sheet of emery onto a flat surface and sit the new winding on the emery and work it around until I have an even track of exposed copper for the carbon brush to pick up.
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Old 14th Sep 2020, 12:21 pm   #2
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Default Re: Rewinding a variac

Wow, that's commitment to the cause. They do come up on ebay from time to time at not enormous prices, but perhaps you feel (I might in your shoes) responsible for its death, and so resurrection Do post updates - I'd like to see this work.
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Old 14th Sep 2020, 4:59 pm   #3
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Default Re: Rewinding a variac

It might be best to ensure your new wire has a high temperature varnish like "Heavy Formvar" rather than the now more common low temperatere self-fluxing solder-through type.

David
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Old 15th Sep 2020, 7:24 am   #4
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Default Re: Rewinding a variac

That's not an easy wind, but its possible. Having wound a few toroids by hand I can recomend having a go, the problem with this variac as opposed to a bog standard toroid is you have to make sure the windings are dressed well and you have X turns per Y diameter consistently over the whole core area, tricky but doable though.

I'd recommend just cutting the old windings off at least for most of the wind, then unwind about an inch to 1/2" to get an idea of windings per inch and how they laid them, then multiply that # by how much diameter is left but add on at least 30 ft. I measure the length of wndgs by going on the local park ( the garden is a no go, too many bushes and things to snag on) & pacing out the distance and pegging one end or whatever then winding onto the shuttle. Take it steady, it's very easy to get kinks.

Lastly Brocott - http://www.brocott.co.uk/enamelled-copper-wire/ is a good source of wire. David makes a good point about coatings, I use the Polyester 200 coated stuff, suitable for Class H. Andy.
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Old 15th Sep 2020, 10:05 am   #5
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Default Re: Rewinding a variac

I always wondered how they achieved the contact area. The wire on my variacs looks as if it has been flattened a bit after stripping, presumably to avoid any possibility of snagging the rotating contact. Very delicate work!
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Old 15th Sep 2020, 1:18 pm   #6
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Default Re: Rewinding a variac

All of the old wire is off, after measuring the length I've settled on approx 197M, this ties in closely with my calculations of 1060 turns. A bit of elbow grease to get the old varnish off and leave a smooth surface, and a bit of work with a small drill to drill out the old wire from the termination holes.

The laminations are a stack of large washers, I guess it makes sense, I wasn't sure what I would find, I have a spool of wire the exact size, it isn't Heavy Formvar insulated type but PE which I think means plain enamel. I do have some other wire that is a bit heavier and I might do some calculations to see how it will fit.

Variacs are able to reduce the voltage but also typically are able to increase the voltage above the input voltage, in this case up from 240v to 260v - this is done with a tap partway along the winding. The mains input is connected to the common and the tap, once the slider reaches and passes the tap the output voltage increases. If I wind thicker wire then the upside is a bit of margin of safety for overloads the downside is the calibrations on the knob will no longer be valid and I won't be able to go the full 260V if I needed it.

The centre of the old winding had a depression in the middle - it looks as if once the winding had been done a plug or boss was clamped on top and slightly compressed to keep the windings tight before they were varnished.
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Old 16th Sep 2020, 7:05 am   #7
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Default Re: Rewinding a variac

"I always wondered how they achieved the contact area" mine looks like the top has been flattened too, there's a definite broadening of the wire.

Was there no insulation covering the core? I can see a bakelite ring on the inside but surely there was some sort of tape or similar covering?

Andy.
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Old 16th Sep 2020, 7:27 am   #8
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Default Re: Rewinding a variac

Its always puzzled me how variacs work, I always understood that shorted turns ARE A BAD THING, yet the carbon brush spans several turns, surely that constitutes a shorted turn?

Peter
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Old 16th Sep 2020, 8:09 am   #9
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Default Re: Rewinding a variac

The planar graphite brushes used in Variacs are specifically chosen to minimise the effect of shorted turns.

Alan
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Old 16th Sep 2020, 8:34 am   #10
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Default Re: Rewinding a variac

The brushes conduct a lot better up/down than right/left. Think of them as a book of carbon layers insulated from each other, current goes up all of them at once, current only goes across (the shorted turn) by going up and then down a few of them. The turns are only shorted a weeny bit, rather clever.
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Old 16th Sep 2020, 8:43 am   #11
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Default Re: Rewinding a variac

Thanks for the explanation. I like that, "The turns are only shorted a weeny bit"

Peter
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Old 16th Sep 2020, 9:03 am   #12
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Default Re: Rewinding a variac

Another image borrowed from Rod Elliott's ESP website illustrating the way in which a Variac's graphite brush minimises the effect of shorted turns.

Alan
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Old 16th Sep 2020, 4:20 pm   #13
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Default Re: Rewinding a variac

I rigged up spindle mounted on some old plummer block style ball races to support the spool of wire, set it up on the rear deck, measured out 5 meters and wound 5 meters at a time onto the shuttle - 200 meters plus a few extra.

I did a trial setup and wound a few turns on to see how it would go - used a small square to make sure the first turn was parallel to the axis of the core - from that I realised that it was quite obvious I would need to rig up something better - I'll need to put some thought into it - the core needs to clamp into a holder of some sort that can rotate so I can see the top and then rotate to see bottom.

The termination board which also carries a fuse holder has two tabs that locate in slots molded into the top and bottom bakelite rings, one of the tabs is shorter than the other so it can be installed after the windings are done.

I always assumed the wire just sanded down enough to make a flat surface for the pick up brush, it did not occur to me to check if the diameter of the wire was greater at the point of pick up brush contact, come daylight I'll have to check, I hope not - I'm not quite sure how I would do that.
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Old 17th Sep 2020, 6:17 am   #14
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Default Re: Rewinding a variac

Tape it with kapton tape.

Andy.
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Old 17th Sep 2020, 4:16 pm   #15
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Default Re: Rewinding a variac

This is what I came up with after an hour or so in the workshop - not very pretty but looks like it will work ok, I can flip it over 180deg and see the top or bottom side, I will initially try to loop 10 or so turns through making sure they are not crossed but not necessarily exactly side by side and then push the turns together/side by side and secure with a drop of super glue and then go again. I'll vary the amount of turns looped through to find a sweet spot where there is a reasonable number of turns that can be pushed together, I'm not keen on only two or three turns at a time, it would take forever.
I've only got polyester tape that I use for transformers, the original start and end of windings were anchored with heavy paper folder over and glued down and then varnished.
I've got a few ideas on getting all of the turns flat and level on the section where the moving contact runs, - I'll wait until the winding is all done and see how it looks first though.
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Old 18th Sep 2020, 10:11 am   #16
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Default Re: Rewinding a variac

I admire your patience. I'd have a go if it was one of the bigger jobs with nice thick wire, but not the finer stuff thanks!
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Old 19th Sep 2020, 10:56 am   #17
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Default Re: Rewinding a variac

The wire is not that thin and may be a bit easier to wind than the thicker variety, it just makes for slow going as I need a lot of turns.

I made a start, the holder works quite well - would be very difficult to do with out it, the inside circumfrence is quite a bit shorter than the outside so the turns need to be laid on top of each other around the centre hole, I need to take as much care in placing the turns around the circumfrence of the centre hole as I do around the outside circumfrence so it is slow going. I'm not glueing the wire as I go - once done I'll make a plug from timber I'll centre on top and and clamp it lightly to put a bit of tension on the turns so they lay flat and then varnish them into position - I hope.
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Old 20th Sep 2020, 8:46 am   #18
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Default Re: Rewinding a variac

Good work, that looks very neat, i can well imagine how much time and care that took.

Andy.
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Old 21st Sep 2020, 8:00 am   #19
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Default Re: Rewinding a variac

Looking great!

Can't wait to see the end result.

Jac
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Old 25th Sep 2020, 10:46 pm   #20
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Default Re: Rewinding a variac

Quote:
Originally Posted by Pamphonica View Post
I always wondered how they achieved the contact area. The wire on my variacs looks as if it has been flattened a bit after stripping, presumably to avoid any possibility of snagging the rotating contact. Very delicate work!
Mine is also definitely flattened along the conract area and I also wondered about that. The exposed copper seems to be coated/plated with some other metal silver in appearance. I was informed by someone on here that this was probably to improve resistance to wear. I imagine it would have to be applied with some kind of plating process after the insulation was stripped and the surface prepared. The brush is made from two separate pieces of graphite material. The unit is rated for 2000W 8A though.

Taking on the task of re-winding a variac does not seem like a job for the faint-hearted so I hope your efforts pay off and likewise look forward to the result!
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